1. Field of the Invention
The present invention relates to a hydraulically damped mounting device.
2. Description of the Prior Art
Mounting devices are known from e.g. our European patent application No. 0115417 and U.K. patent application No. 84.20035 having two anchor points connected together by a resilient spring, the resilient spring defining one wall of a working chamber for hydraulic fluid connected by an elongate passageway to another chamber. Relative movement of the anchor points causes a change in the volume of the chamber bounded by the resilient wall. This forces the fluid through the passageway and the viscosity causes damping of the movement.
Such mounting devices are used, for example, to mount a car engine in the body so that vibration of the engine is damped before it is transmitted to the car body. For suitable mounting three or more such mounting devices may be provided to support the engine at separate positions. However, it has been found that this arrangement for damping the vibration of the engine is unsatisfactory for torsional movement. Since each mounting device is independent, different damping effects may result at different points of the engine and this may accentuate, rather than diminish, torsional movement. This is a particular problem because it is desirable that the natural frequency of the mounted system under torsional vibration is high and this may result in the natural frequency being excited by the engine vibration, causing resonance.
Therefore the present invention proposes a mounting device with at least two working chambers interconnected by an elongate passageway and held apart by an elongate rigid member. The result is a hydraulically damped tie-rod which may be interconnected between one vibrating body and another (e.g. between an engine and the car body) which resists torsional movement.
Preferably the mounting device has two bushes connected together by a rigid bar or rod. Each bush has a resilient spring which connects the bush to an anchor point and which acts as one wall of a working chamber within the bush. The working chambers are connected together by an elongate passageway which conveniently may be formed by a bore in the rod.
Each spring may also bound a second working chamber filled with hydraulic fluid, which chambers are connected by a second passageway. This increases the damping force and makes for a more symmetrical construction because each vibration then causes a compression of one of each pair of chambers and an expansion of the other pair. When there is only one pair of chambers, the spring attached to anchor point is compressed while the other expands. The effect of this may be to create unequal damping.
The chambers are normally arranged so as to resist forces causing compression or tension in the rod.
One suitable arrangement has each bush in the form of a hollow unit with the anchor point approximately centrally within the bush and the spring extending from the anchor point to the surrounding walls of the bush, so that the working chamber is bounded by the spring and by a part of the inner wall of the bush. When there is only one working chamber within each bush, the rest of the interior of the bush may be filled with air, which is preferably vented to the atmosphere to prevent temperature changes causing a change in characteristics. When there are two working chambers within each bush the spring may divide the interior of the bush into two, so that the two chambers fill the bush.
By suitable selection of the length and cross-sectional area of the passageway the damping effect may be adjusted so that the damping is optimised at the resonant frequency of torsional vibrations.
In order to improve the high frequency damping effect it would be possible to provide gas pockets within the working chambers, the gas pockets being bounded by a diaphragm as in EP-A No. 84.300406.0.
The resilient spring is preferably made of rubber or similar material. However, such materials should not be under tensile loads. Therefore it is desirable that the resilient spring is pre-compressed during the manufacture of the mount so that it remains under compression during all vibrations.